1. Field of the Invention
This invention relates to a gallium nitride-based compound semiconductor light-emitting device which is able to emit blue light or light in a short wavelength spectral range. The invention also relates to a method for making the device.
2. Description of Related Art
Light-emitting diodes using GaN-based compound semiconductors (Al.sub.x Ga.sub.1-x N wherein 0.ltoreq.X&lt;1) are known as ones which are able to emit blue light or light in a short wavelength spectral range. Attention has been now directed to the GaN-based compound semiconductors because they come in direct transition so that a high light emission efficiency is attained, and are able to emit blue light which is one of the three primary colors.
With such GaN compound semiconductors, low resistance p-type crystals are not obtained. In general, a light-emitting diode using a GaN compound semiconductor is arranged to have a so-called MIS structure which includes a metal electrode, an i-type layer (insulator) made of semi-insulating GaN and an n layer made of n-type GaN. Light emission takes place at a portion beneath the electrode (light emission electrode) on the i-type layer. More particularly, the electrode-forming portion has the MIS structure.
In a GaN blue LED having an MIS structure as mentioned above, it is important that the device structure and the layer arrangement be established firsthand in order to have light emitted efficiently.
In light-emitting devices having a pn junction structure wherein other compound semiconductors of groups III-V, such as Al.sub.x Ga.sub.1-x As are used, an electric current is diffused transversely along the interface of the junction in the device and, thus, the current passes vertically and uniformly with respect to the interface. As a consequence, unlike an MIS-type LED wherein light is emitted only at a portion beneath the electrode, light is emitted from the entire interface irrespective of the size of the electrode. Because the light is substantially uniformly emitted from the interface, pickup of light is easy.
However, with a GaN blue light LED having an MIS structure, little current diffusion along the transverse direction parallel to the interface takes place in the i-type layer beneath the light-emitting electrode. This results in a light-emitting portion which is limited only to a region beneath the light-emitting electrode. Because the electrode is generally made of a metal, light emission is rarely observed from the side of the light emission electrode as if disappearing behind the electrode.
To avoid this, known GaN blue light LEDs make use of a sapphire substrate and GaN, both of which are transparent to emission light. More particularly, it is customary to utilize a flip-chip system wherein a light emission electrode is provided at the lower side of the substrate or, instead, is provided in a system wherein light is picked up from the back side through the substrate. To this end, a light emission electrode and an electrode electrically connected to an n-type layer (an electrode at the side of the n-type layer) are formed on the surface of a GaN epitaxial layer. These electrodes are bonded with a lead frame by means of a solder, making it possible to pick up light through the substrate.
However, when using the flip-chip system wherein a light emission electrode (i-type layer electrode), an n-type layer electrode and a lead frame are bonded through a solder, the electric series resistance component of the device has to be increased for the following reasons:
(1) Because the distance between the electrodes cannot be made too narrow in order to prevent short-circuiting the light emission electrode (i-type layer electrode, n-type layer electrode and the solder), the electric resistance component becomes large.
(2) If the light emission electrode (i-type layer electrode) and the n-type layer electrode greatly differ in shape under which a solder bump is formed, the solder bumps have inevitably different heights, so that a connection failure with the lead frame will be likely to occur.
Accordingly, it is necessary to shape the electrodes so as to have substantially the same area. This leads to a loss in the degree of design freedom of an electrode pattern, further resulting in difficulty in obtaining an optimum pattern for reducing the electric resistance component. The large electric series resistance component not only lowers the light emission efficiency, but also unfavorably induces generation of heat in the device which causes device operation to degrade and light emission intensity to become lower.